This disclosure generally relates to systems for carrying tools across surfaces, such tools including (but not limited to) sensors used in nondestructive evaluation (NDE). In particular, this disclosure relates to tool-carrying crawler vehicles that are capable of operating on a non-level surface.
Prior art systems for inspecting a non-level surface, such as the surface of an aircraft fuselage, include track-based systems, large robotic manipulator arms to position the tool, hand-held scanners and differential-drive crawler vehicles. The term “differential drive” refers to a type of vehicle motion control that makes turns by independently driving the wheels on opposite sides of the vehicle. In vehicles with four or more wheels, this type of motion control is sometimes called skid steer. Such vehicles are subject to motion constraints (i.e., they do not allow simultaneous translation and rotation) and are considered to be non-holonomic-motion systems.
It is known to use a non-holonomic-motion crawler vehicle to position and move NDE (nondestructive evaluation) scanning equipment on an aircraft fuselage. The scanning process requires precise position and orientation control in order to achieve the desired scanning path. Standard differential-drive vehicles will tend to slip sideways slightly when external forces are applied perpendicularly to the direction of travel. In the case where a surface of an airplane fuselage is being scanned, the vehicle needs to be attracted to the surface—usually with some type of suction or vacuum creation system. As the vehicle moves horizontally on and across the side of the fuselage, the external force of gravity pulls on the vehicle, causing it to slip sideways. Since a differential steer vehicle cannot directly control sideways motion, NDE scans acquired with that type of vehicle (i.e., a non-holonomic platform) can be distorted.
If a holonomic vehicle were to be used instead, any undesired motion could be corrected directly. The most common type of holonomic vehicle configuration uses a type of wheel called a Mecanum wheel. A Mecanum wheel is a type of wheel with multiple individual rollers that, when used in pairs, allows vehicle motion in any direction (i.e., holonomic motion). While these wheels work well on level surfaces, they have trouble creating the desired motion on inclined surfaces. This problem is due to the requirement that all wheels have sufficient traction on each wheel to support the forces required to make the desired movement. This is especially true of sideway motions.
There is a need for a system that enables precise control of the holonomic motion of a tool-carrying vehicle on a non-level surface.